Sept.,1956 371 RED-TAILED HAWK AND HORNED OWL POPULATIONS IN WISCONSIN By GORDON ORIANS and FRANK KUHLMAN In the winter of 1953 we began a three-year investigation of interactions between populations of the Red-tailed Hawk (Buteo jamaicensis) and the Horned Owl (Bubo virginianus) on 9.5 square miles of open farm land in southern Green County, Wisconsin. The study area included all of Sylvester and Jefferson townships and parts of Clarno and Monroe townships. Additional observations were made on land within one-quarter mile of the area since birds &sting there fed partly in the study area. In the three-year period 90 nests of Red-tails and 48 nests of Horned Owls were examined. The original vegetation of the area, as determined from surveyors’ records by Dr. J. T. Curtis, Department of Botany, University of Wisconsin, consisted of a tongue of maple (Acer saccharum)-basswood (Tiliu americana) forest bordered on the east and northwest by a belt of oak-basswood (Quercus-Tilia) forest which continued acrossthe north-central portion of the area. The southeast part and most of the northern edge of the a.rea were covered primarily by tall grass prairie with small islands and extensions of the forest. Extensive agriculture has greatly altered this original vegetation so that only small woodlots remain (fig. 1). Maple-basswood has replaced the oak-basswood chiefly on the north slopes, but oak dominates the drier slopes.The prairie areas are still essentially treeless with only a few scattered fence-row willows (Sal&), American elms (Ulmus americana), and black cherries (Prunus serotina) (fig. 2). The principal farm crops are corn, hay, and oats. Much of the land is in pasture. The other avian raptors breeding on the area were Screech Owls (O&s asio), a few Sparrow Hawks (Falco sparverius), and Cooper Hawks (Accipiter cooperi). Broad- winged Hawks (Buteo pZutypterus), Sharp-shinned Hawks (Accipiter striatus) , and Marsh Hawks (Circus cyaneus) visited the area during migration, and Short-eared Owls (Ash flammeus) were uncommon in winter. Rough-legged Hawks (Buteo Zago- pus), sometimes as common as Red-tails between mid-November and mid-April, were the only other large raptors on the area in any numbers. The word “competition” has been used to refer to a confusing variety of relation- ships between individuals, but recently Andrewartha and Birch (1954) have given good reasons for restricting the use of that term. Therefore, in this paper we have adopted the following terminology: Interaction, any relationship among individuals of the same or differing species; competition, the demand of individuals upon any common resource which is in short supply; and interference, any detrimental interaction not falling into the class of competition as just defined. ACKNOWLEDGMENTS We wish to expressour gratitude to Tom Soulen, John Kaspar, and George DeCour- sey of the University of Wisconsin who helped with some of the field work. Mrs. Orians also aided in the field work, typed the manuscript, a.nd offered many valuable sugges- tions during the preparation of the paper. Discussions with Dr. David Lack of the Edward Grey Institute of Field Ornithology, Oxford, and Dr. Dean Amadon of the American Museum of Natural History, New York, were helpful in the initial prepara- tion of the report. The help of Dr. John T. Emlen of the University of Wisconsin was invaluable throughout the field work. He also offered valuable criticism of the manu- script. To all these people we are grateful but we take full responsibility for any. errors and omissions and for interpretation, for we have not always followed their advice. The Department of Zoology, University of Wisconsin, furnished the vehicle used for much of the field work. 372 THE CONDOR Var. 58 THE RED-TAILED HAWK POPULATION Winter population.-Throughout the winter the Red-tails moved about consider- ably, often sitting and hunting in small groups. We encountered birds throughout the area but most frequently in the wooded portions. Rough-legged Hawks, on the other hand, were most common on the prairie areas. Of considerable interest was the scarcity of immature Red-tails in the winter. From November 30, 1953, to February 15, 1954, we observed 39 adults and 10 immature Fig. 1. Typical scene on study area showing small woodlots, scattered trees along a stream, and cropland. birds. In a comparable period the following winter, we saw 36 adults and two immatures. Many immature birds may migrate to the Gulf coast, as we have three winter recoveries from this region of nestlings banded in Green County (table 1) . In early autumn we often saw paired birds over their nesting woods, but this was not observed in the winter. Yet Fitch, Swenson, and Tillotson (1946) found that Cali- fornia birds were paired and permanently resident in definite hunting and nesting ter- ritories. On our study area all resident birds were paired on their territories by the end of February and only migrants travelled singly or in groups. Breeding population.-The huge, relatively flat nests of the Red-tail, built on large horizontal branches, often at considerable distance from the trunk, were easily located in winter and readily identified. We noted the location of all old nests in the winter since many of them would be repaired or new ones built upon their foundations. New nests apparently were built in a few days. We observed birds building only twice. On January 30, 1952, we saw two birds carrying sticks to a nearly completed nest and watched building at another nest on March 6, 1954. Twigs used in new nests were obviously broken from larger branches, as the fresh ends were conspicuous. Corn husks were used as lining material in all nests we studied. Although nests were sometimes built as early as late January, incubation did not start at such an early date. We flushed our earliest incubating bird from a nest on March 1, 1953. We found the first incubating birds in 1954 and 1955 on March I3 and 11, respectively. By aging the young at the time of banding and using an incubation Sept., 1956 HAWK AND OWL POPULATIONS 373 period of about 35 days (Hardy, 1939), we estimated that the first eggswere laid in the first week of March but that most of them were laid in the second and third weeks. Both sexes participated in incubation. Twice we observed the replacement of one bird by the other. None of the nests under observation was used three years in successionby Red-tails, but ten nests were used two years in succession.Eight new nests were built in the same woods as the old one, and sometimes the Red-tails re-used a nest after it had remained Table 1 Recoveries of Nestling Red-tailed Hawks Banded on the Study Area Distance and direc- tion fro;ati;;ding Date banded Date recovered Place recovered May 26, 1951 July 25, 1951 Monroe, Wis. 3 months May 10, 1952 Dec. 23, 1952 San Augustin Co., Texas 8 months 800 miles SSW May 12, 1952 March 9, 1953 Brownview, Point Coupe 10% months 850 miles SSW Parish, La. May 2, 1953 Dec. 1, 1953 Jersey, Buchanan Co., Iowa 7 months 120 miles W May 2, 1953 Nov. 3, 1953 Elgin, Kane Co., Illinois 6 months 80 miles SE May 8, 1953 Aug. lo, 1953 Dakota, Waushara Co., Wis. 3% months 110 miles N May 9, 1953 Sept. 4, 1953 Basco, Dane Co., Wis. 4% months 25 miles N May 9, 1953 Dec. 27, 1953 Lake City, Columbia Co., Fla. 8 months 950 miles SE May 23, 1953 Jan. 30, 1954 Evansville, Rock Co., Wis. 9 months 23 miles NE May 6, 1954 July 27, 1954 Juda, Green Co., Wis. 2% months 2 miles NE May 20, 1954 July 13, 1954 Stevenson, 111. 2 months 5 mile S May 20, 1954 Dec. 7, 1954 Fairbury, Livingston Co., Ill. 7 months 120 miles SE May 22, 1954 Sept. 3, 1954 Monroe, Wis. 3% months 2 miles W unused for one season. Often Horned Owls used the old Red-tail nests, and sometimes the two speciesused the same nest in alternate years. Although elms were outnumbered by both sugar maples and white oaks (Quevcus &a), 52 of the 90 nests we found were in elms, perhaps because these trees are com- mon along streams and fence-rows, and because their large, spreading branches provide many suitable nest sites. We found 15 nests in sugar maples, 9 in white oaks, 6 in bass- woods, 5 in willows, 2 in red oaks (Queycus borealis), and one each in burr oak (Quercus macrocarpa) , black walnut (Juglans nigra) , and cottonwood (Popuhs deltoides). The height of 49 nests ranged from 30 to 90 feet and averaged 57 (k13.1) feet. Nine nests were enclosed by dense woods so that the adults had to fly through trees to reach them. Twenty-three were on the edges of dense woods with accessfrom the open air to the nest (fig. 4~). Fifty-seven nests were situated in isolated trees or in open stands of four to several dozen trees with unobstructed accessto the nest from several sides (fig. 4b). Because Red-tails will nest in such open situations, the prairie regions of our study area were inhabited by them (fig. 3 ) . In 1954 there were 33 nesting pairs and six non-nesting pairs (0.41 pairs per square mile), while in 19.55 there were 27 nesting pairs and one non-nesting pair (0.29 pairs per square mile). Only after a careful search of the area and a study of the birds’ be- havior did we call pairs non-nesters.